The present invention relates to a method of cryogenic fractionating and purification of gas.
It is also directed to a heat exchanger for carrying out this method.
Some gases comprise at the same time components which are rather easily liquefiable at low temperature and components which are liquefiable with more difficulty or non-liquefiable. It is therefore usual to attempt to separate them by cooling to condensate the easier liquefiable elements and to thus separate them from the components which are liquefiable with more difficulty or non-liquefiable.
Among the gases with several components which may thus be processed may be mixtures of different hydrocarbons or with non hydrocarbonic components such as nitrogen, hydrogen, argon and/or carbon monoxide and for example the gases from catalytic cracking or steam cracking.
To achieve the required cooling, use is made in the prior art of heat exchangers and in particular of reflux exchangers also referred to as "dephlegmators", the external refrigeration being usually supplied in counter-current relationship by a refrigeration cycle or by a dynamic gas expansion. This would limit the use of these techniques to temperatures at which the refrigeration cycles are available and to the cases where the expansion of the effluents for example of hydrogen or methane is possible.
It is also possible to use a self-refrigeration process. The process consists in cooling the gas to be purified in a first exchanger, in separating the non-condensed gas from the first condensate formed, for example in a fractionating column, and further cooling the non condensed gas in a second exchanger to form a second condensate, in separating this second condensate from the non-condensed gas in a separator and to return the second condensate to the column as a reflux.
The non-condensed gas separated from the second condensate constitutes the purified gas. The coolant for both exchangers is constituted by the first condensate which is subjected to a vaporization through expansion and flows successively through the second and then the first exchanger. The purified gas may itself flow through the second and then the first exchanger.
The method and the device according to the invention exhibit the advantage of not requiring as a general rule a refrigeration by means of refrigerants extraneous to the equipment and of not requiring any expansion of that or those component(s), liquefiable with more difficulty, of the treated gaseous mixture. The latter point is important since on the one hand the liquefaction processes most often require the application of a high pressure and on the other hand some separated gases obtained, such for example as hydrogen and/or carbon monoxide often are reagents for chemical reactions which have to be operated under a high pressure. It would therefore not be very economical to expand these gases during the cryogenic separation in order to have then to recompress them.
Furthermore the method and the device according to the invention are more economical than the known self-refrigeration method since they require one unitary exchanger only which is cheaper than the multiple appliances (at least two exchangers, one fractionating column, one separator and many circuits) of the known process. They also would reduce the thermal losses and avoid high expenditures for the insulation of the circuits and apparatus.
The gases to which the invention applies are mixtures of at least two and preferably of at least three different chemical components with different boiling (or condensation) temperatures under the conditions of the method and for example a mixture of hydrogen, of methane and of at least one C2-hydrocarbon such as ethane or ethylene with or without higher (C3 or more) hydrocarbons. Other mixtures in addition include carbon monoxide and/or nitrogen.
The method according to the invention is a self-refrigerated method of cryogenic fractionation and purification of a gaseous feed fluid with at least two components condensable at different condensation temperatures, namely at least one relatively heavy component to be removed and at least one relatively light component to be recovered, respectively, so as to produce a purified gas preferably comprising the relatively light component(s) and a separated gas preferably comprising the relatively heavy component(s), characterized in that it consists in operating in a heat exchange zone forming a unitary assembly and comprising at least five distinct aggregately vertical circuits referred to as the first, the second, the third, the fourth and the fifth circuits, respectively, in indirect heat exchanging relationship with each other at each level of the heat exchange zone, the first circuit or reflux circuit being essentially arranged within an upper and relatively colder portion of the heat exchange zone and the fifth circuit being essentially arranged in a lower and relatively less cold portion of the heat exchange zone, which method comprises the steps of circulating at least one fraction of the gaseous feed fluid aggregately from bottom to top within the fifth circuit under such conditions that it may condense in part to give a first condensate and that this first condensate be carried along without any substantial reflux by the said gaseous fluid, discharging the resulting mixture of non condensed gas and of the first condensate from the top of the fifth circuit, separating the said non condensed gas from the first condensate in a phase separation zone, circulating the gas thus separated aggregately from bottom to top in the first circuit or reflux circuit under such conditions that one part of the gas may give a second condensate and that this second condensate may flow back in the said first circuit and be collected at its bottom, circulating at least one part of the non condensed gas discharged from the top of the first circuit aggregately from top to bottom in the second circuit in counter-current relationship with the fluid circulating in the first circuit and then with the fluid circulating in the fifth circuit, and discharging the resulting purified gas, circulating the first condensate and the second condensate aggregately from bottom to top in at least one third circuit to there undergo a sub-cooling, discharging from the top of the third circuit the resulting sub-cooled first and second condensates, expanding them and circulating them aggregately from top to bottom in at least a fourth circuit where they are vaporized upon taking heat from the fluids of the first, third and fifth circuits, at last discharging the said vaporized condensates from the bottom of the fourth circuit, these vaporized condensates constituting the separated gas.
Thus the invention operates a unitary heat exchanger (a unitary heat exchanging zone) comprising over at least one part of its height, at least five circuits, each one preferably of the multi-channel type, aggregately directed vertically. One of the circuits called reflux circuit or first circuit is essentially arranged within an upper portion of the exchanger (the exchange zone), i.e. within a relatively colder portion of the exchanger. This preferably is a "non tortuous" circuit, i.e. wherein the condensed liquid may stream in one aggregately downward direction. Another circuit (fifth circuit) preferably of the tortuous kind unfit for liquid reflux is essentially arranged in a lower portion of the exchanger (the exchange zone), i.e. in a relatively less cold portion of the exchanger.
With an aggregately vertically directed circuit of the non tortuous type is meant a circuit such that the fluid, which is fed thereinto at the bottom, may flow forward in a general manner from bottom to top without any substantial reflux of the liquid portions of this fluid, whereby there is supposed to be for example a smaller mean slope or gradient than in the aforesaid reflux circuit; in other words, all or almost all of the (liquid and gaseous) fluid will follow an aggregately upward directed path in this circuit of the tortuous kind and will be collected at the top of the said circuit, the discharge point (or zone) being located in an intermediate portion of the heat exchanger, for example in the vicinity of the first third or of the half-height of the exchanger.
It is preferable that the aforesaid tortuous circuit be entirely or almost entirely at a lower level than the reflux circuit and still better that both circuits be arranged substantially above each other in the exchanger.
The second, third and fourth circuits may either be or not be tortuous, preferably non-tortuous.
It is however not indispensable to use a tortuous circuit and a non-tortuous circuit to achieve the results (reflux and non reflux, respectively), referred to hereinabove. One may indeed act upon the cross-section of the circuit and/or the flow velocity of the feed fluid in this circuit. A low speed within a relatively wide channel permits the reflux indeed whereas at a high speed within a relatively narrow channel results in the condensate being carried along thereby preventing it from flowing back. A multi-channel circuit with a small cross-section and a great flow velocity therefore is advantageous in particular for the fifth circuit. The five aforesaid circuits are in heat exchanging relationship with one another at each level of the exchanger where they are present, thereby assuming that the exchanger is preferably made from a good heat conducting material with walls with the smallest possible thickness compatible with the strength of the materials and comprising a large exchange surface. Those skilled in the art will be able to make such exchangers without any difficulty from the foregoing statements.